1. Field of the Invention
The present invention relates to driving devices for driving image display elements by use of modulation pulses as modulated based on luminance data. More particularly but not exclusively, this invention relates to driving devices adaptable for use in image display apparatus equipped with an image display unit having a plurality of image display elements wired together into a matrix.
2. Description of the Related Art
Image display apparatus using image display elements including electron emitting elements and electro-luminescent (EL) elements has been studied. This type of image display apparatus is more excellent in characteristics than other types of conventional image display apparatus; so, the demand therefor is expected to rise in near future. For instance, the image display apparatus is advantageous over recently widely used liquid crystal display (LCD) devices in that the former requires no back-light units because of self-luminous type, and also in that the former is wider in viewing angles than the latter.
FIG. 28 indicates schematically one example of a multiple-electron source unit using electrical wiring methods. This multi-electron source unit is arranged to include a number of electron emitting elements which are laid out two-dimensionally so that these are electrically wired to have a matrix form as shown in the drawing.
In FIG. 28, reference numeral “1”designates electron emitting elements which are represented by symbols; numeral 2 denotes row wirings; 3 shows column wirings. The row wirings 2 and column wirings 3 have electrical wiring resistance 4, 5, wiring inductance 6, 7, and wiring capacitance 8. Numeral 9 denotes a scanning circuit; 10 is a modulator circuit; 11, a multi-electron source substrate; 12, a substrate.
In the multi-electron source unit with the matrix-wired electron emitting elements, appropriate electrical signals are applied to row and column wirings in order to output a desired electron beam.
FIG. 29 is a schematic waveform diagram for explanation of a pulse width modulation (PWM) scheme. For example, in order to drive any given row of electron emitting elements in the matrix, a selection potential Vs is applied to a row wiring of a presently selected row and a non-selection potential Vns, at the same time, is applied to row wirings of non-selected rows. In a way synchronous with this voltage application, a drive potential (modulation pulse signal) Ve is applied to a column wiring for output of an electron beam.
With this method, a voltage of Ve–Vs is applied to an electron emitting element in the selected row, while a voltage with a potential Ve–Vns is applied to an electron emitting elements in the non-selected rows. Setting the voltages Ve, Vs and Vns at appropriate potential levels enables an electron beam with a desired intensity to be output from only the electron emitting element or elements in the selected row. Additionally in view of the fact that cold cathode elements are inherently high in speed of response, adequately varying the length of a time period for application of the drive potential Ve makes it possible to change the length of a time period for electron beam output.
Similar electron beam controllabilities may also be achieved by other techniques such as pulse wave peak value modulation schemes, also known as pulse height modulation (PHM), which control the luminance by changing the potential level (amplitude) and/or current value of a modulation pulse being applied to a column wiring.
Unfortunately, currently available large-screen high-definition image display apparatus with enhanced fidelity and increased resolution capability—such as displays with 1,920 by 1,080 dots of effective pixels and a frame rate of 60 hertz (Hz) and also 10-bit gradation tonality—are encountered with problems which follow.
Letting the wave height or peak value of an energy applied to an element in pulse peak value modulation schemes be Pi, the above-noted image display apparatus requires a resolution of Pi/210=Pi/1024. Pi is set at several volts (V) in the case of voltage driving and, therefore, a resolution of several millivolts (mV) is required for a drive waveform over the entirety of a display screen with 1,920-by-1,080 pixels. However, in view of electrical characteristics of components making up drive circuitry such as integrated circuit (IC) chips and printed wiring boards and power supply units, it remains difficult to achieve such level of resolution.
On the other hand, in the case of pulse width modulation schemes, a time taken to drive a single scan line is 1/(60×1080) sec, which is nearly equal to 15 microseconds (μsec). In case 10-bit pulse width modulation is performed, the minimum pulse width is 1/(60×1080×210) sec. which is about to 15 nanoseconds (nsec). In this case the pulse width resolution of 15 nsec in minimum is required.
However, the wirings as shown in FIG. 28 are equivalent to low-pass filters having a cut-off frequency that is determined by the wire inductance (L), wire capacitance (C) and wire resistance (R). Accordingly, in the case of driving signal transmission wirings and/or display unit wirings having such low-pass characteristics by line-sequential pulse width modulation (PWM) drive schemes with frequency spectrum components higher than or equal to the cutoff frequency, PWM waveforms being applied to elements can experience unwanted rounding of the rising and falling rectangular edges thereof as shown in FIG. 30. This wave edge rounding would result in a decrease in display quality at low luminance levels.
In particular, in case a PWM drive waveform with low gradation is applied from the modulator circuit 10, a synthetic waveform that is created by combination of a drive waveform and an output waveform of the scanning circuit 9 and is applied to an electron emitting element 1 decreases in peak value. This leads to a decrease in peak value of a drive waveform that is made up of high frequency spectrum components only, that is, the low-gradation PWM drive waveform. In other words, it is no longer achievable to visually display any image with desired tonality in low gradation regions.
Also note that in the case of supplying constant current pulses with short time lengths from a control constant current source to a multi-electron source unit with a very large number of matrix-wired electron emitting elements, a problem that electrons are hardly released in any way arises. Obviously, electrons are emitted in case constant current pulses are continuously supplied within a relatively long time period; however, a lengthened rise-up time must be required until initiation of electron emission.
FIG. 30 is a timing chart for explanation of the problems above. As shown herein, when merely supplying short current pulses from the control constant current source, a drive current If hardly flows in any electron emitting element. Even when supplying long pulses, the drive current If flowing in the electron emitting element comes to have a waveform with an increased rise-up time. This occurs due to some rounding of rectangular-wave edges of the current waveform being fed to the electron emitting element. More specifically, the current waveform can experience such edge rounding in spite of the fact that the cold cathode type electron emitting element per se has high-speed response performance. Such waveform rounding results in deformation or distortion of the waveform of an emission current Ie.
In multi-electron source unit with passive matrix-wired electron emitting elements, the parasitic capacitance (wiring capacitance) increases with an increase in matrix scale or size. Main part of the parasitic capacitance is present at an intersection between row and column wirings. An equivalent circuit of this is shown in FIG. 28. When beginning supply of a constant current I1 from the modulator circuit 10 for use as the control constant current source that is connected to column wirings 3, this current is consumed for charge-up of the individual parasitic capacitance 8 early in the time period and thus hardly acts as the drive current of an electron emitting element or elements 1. Due to this, an appreciable decrease takes place in effective response speed of the electron emitting element(s).
Additionally the voltage drive schemes are faced with problems to be solved as follows. Image display apparatus using light emitting elements of the type causing a current to flow during driving—for example, light-emitting diodes (LEDs), electroluminescence (EL) devices, field emission display (FED) elements, surface-conduction electron-emitter display (SED) elements—is generally designed so that the wiring resistance is set lower in value. Accordingly its equivalent circuitry is given as the model shown in FIG. 28, which comes with the parasitic capacitance, resistance, and inductance components. When applying the related art voltage drive method to such circuitry, a drive waveform experiences some rounding of rising edges due to the flow of a charge-up current i into the parasitic capacitance upon application of a voltage. Furthermore, owing to the self-inductive action of the parasitic inductance, an electromotive force of U=−L×(di/dt) is produced resulting in generation of over-shooting and/or ringing, which leads to unwanted occurrence of application of abnormal voltages to light emitting elements.
In recent years, image display apparatus is under growing requirements for achievement of larger screen sizes and higher precision and also higher gradation. The quest for larger screen sizes and higher precision and higher tonality results in increases in parasitic inductance and capacitance of electrical wires used. Hence, the problems such as the failure of tonality in dark regions due to the rounding of rising edge of the drive waveform, the overshoot and the ringing are becoming more serious issues to be solved.
Another problem of the approach using drive waveforms created by means of plain pulse width control and pulse peak value control schemes is that uniform increasing characteristic of the grayscale/gradation tonality is no longer guaranteeable due to any possible changes and fluctuations of the voltage versus luminescence intensity characteristics of light emitting elements.
In addition, drive waveforms based on the simple pulse width control are such that their pulses are made identical in start timing as shown in FIG. 31. With such aligned pulses, a large current rushes to flow in a scan wiring at the time of potential rise-up of a pulse width modulation waveform, resulting in occurrence of a voltage drop. In a multi-electron source unit with a great number of matrix-wired electron emitting elements, the voltage to be applied to each element decreases due to the voltage drop occurring by the influence of the resistance component of such wiring. The greater the distance from its power supply end, the lower the element-applied voltage. A result of this is that the emission electron distribution pattern of each element fails to stay uniform. And,when applying such multi-electron source unit to image display apparatus, a problem arises that the display quality can decrease due to the voltage drop as created by the wiring resistance.
An explanation will be given of the voltage drop with reference to FIG. 28 and FIG. 32. FIG. 32 is a perspective view of an image display panel using the multi-electron source substrate 11 of FIG. 28. In FIG. 32, numeral 13 designates a metal back plate; 14 denotes a fluorescent layer; 15 is a front face plate.
Now suppose that a row wiring 2 is selected. Assume that all of the pixels connected to the selected row wiring 2 are driven to turn on. An equivalent circuit at this time is shown in FIG. 33. In this drawing, 16 denotes current components flowing into the row wiring 2 from its associated column wirings through corresponding electron emitting elements; 4 indicates resistance components of row wiring 2.
Here, assume that the currents flowing into the row wiring 2 measure the same value If for respective elements. Also assume that the resistance value of row wiring 2 per pixel is rf. A voltage potential on the row wiring at this time is calculated in a way which follows.
A current which flows in a resistance component Rf5 is If. A voltage drop caused by Rf5 is given as If·rf. A current flowing in Rf4 is 2·If, and a voltage drop by Rf4 is 2·If·rf. Similar calculations are repeated to determine the voltage drop at each resistance component. Calculation results of the potential at each portion on the row wiring 2 are plotted in a graph of FIG. 34. Note here that the data plot in this graph is under an assumption that the drive potential Ve is higher than the selection potential Vs, i.e. Ve>Vs.
Remarkably, when the potential Vs is output from the scanning circuit 9 that is a power feed point, a current flows into the row wiring 2, resulting in an increase in potential with an increase in distance from the power feed point. Especially a potential increase at the farthest end is as large as 21·If·rf. FIG. 35A shows the waveform of a voltage signal which is applied to a row wiring; FIG. 35B shows a drive waveform that is applied to a row wiring at the farthest end; and, FIG. 35C is a voltage waveform as applied to a selected electron emitting element. It can be seen that the potential riseup causes the selection potential to change from Vs to Vs′, resulting in a likewise decrease in element-applied voltage.
This voltage difference pauses no particular problems in case the row wiring stays less in resistance. However, such voltage difference is no longer negligible in some cases—for example, when the row wiring has an increased value of resistance due to an increase in screen size of image display apparatus. The voltage difference also becomes greater in cases where the pixels used increase in number resulting in an increase in current flowing into the row wiring.
This voltage difference causes electron emitting elements to differ from one another in voltage applied thereto. In particular, an electron emitting element near the power feed point and another element far therefrom are such that the same voltage is never applied thereto, resulting in occurrence of an appreciable difference therebetween in electron emission amount. This is observable as a luminance difference between pixels, which leads to a decrease in display quality.